I swore I would never touch an MMO.........sigh......well the Star Wars story is just too good so yeah.......
I'm now rolling a level 38 Sith Juggernaut (had to go imperial......no way am I rolling republic scum! hehe).
pacov, you gain more credits from completing ALL bonus mission objectives as well as keeping your credits by not dying. Everytime you die your armour takes a huge hit and if you need to repair it at a vendor you can have a big bill really fast. For instance after one particularily hard boss battle and many deaths the repair bill for my healer companion and myself (I had solo'd that quest) was just shy of 7,000 credits.
Also, as pointed out.......I can't even get a new skill these days for under 15-20K. Isn't really a problem though cause I've been saving for the speeder upgrade at level 40 and currently have about 300,000 credits saved.
Make sure you do missions with your companions and make sure your companions LIKE you (as in keep working at maxing out their "affection") since your mission results with companions will yield better results the more they like you.